The present invention relates to a method for determining parameters for the viscosity and/or temperature of a brake fluid of a vehicle.
It is known that the, viscosity of a brake fluid or hydraulic fluid is greatly responsive to temperatures. High viscosity at low fluid temperatures, i.e., at a low temperature below xe2x88x9210 degrees C., for example, in the starting period of an automotive vehicle impairs the controllability of the brake pressure of a controlled hydraulic brake system. It is problematic when brake fluid, e.g. within a driving stability control function, i.e. without being influenced by the driver, shall be conducted especially quickly from the brake fluid reservoir to a wheel brake. With temperatures dropping, the viscosity of the brake fluid rises overproportionally. At very low temperatures, the result is that the brake fluid cannot be aspirated at a sufficiently quick rate and, in addition, the loss in pressure in the pipe line increases with rising viscosity. These obstacles cause a decelerated brake intervention. In driving stability control, however, there is the general demand of effecting a quick brake intervention. To solve this problem, devices have already been proposed which provide an auxiliary pressure source or a precharging pump (WO 96/20102). Because this entails considerable extra cost, the use of these devices is increasingly avoided.
In view of the above, an object of the present invention is to ensure, with little effort, the functioning of a hydraulic vehicle brake system with all its partial functions, such as anti-lock function, traction slip function, and driving stability function, even when exposed to very low outside temperatures.
Favorably, the method is used in a driving-dynamics control system which serves to assist the driver of a vehicle in critical driving situations. xe2x80x98Vehiclexe2x80x99 in this context refers to an automotive vehicle with four wheels which is equipped with a hydraulic brake system. In the hydraulic brake system, brake pressure can be built up by the driver by means of a pedal-operated master cylinder. Each wheel has a brake with which at least one inlet valve and one outlet valve is associated. By way of the inlet valves, the wheel brakes are connected to the master cylinder, while the outlet valves lead to an unpressurized reservoir or low-pressure accumulator. Finally, there is provision of an auxiliary-pressure source, generally, a motor-and-pump assembly which is able to build up pressure in the wheel brakes even independently of the position of the brake pedal. The inlet and outlet valves and the further valves arranged in the brake circuit are electromagnetically operable by actuation of valve coils for the pressure control in the wheel brakes. Four rotational speed sensors, one per wheel, one yaw rate sensor, one transverse acceleration sensor, one steering angle sensor, and at least one pressure sensor for the brake pressure generated indirectly or directly by the brake pedal is provided in order to detect conditions related to driving dynamics. An electronic control system which typically forms a construction unit along with a hydraulic block, in which the valves and the pump are accommodated, and on the one side of which the pump motor is arranged, controls the dynamic driving conditions of the vehicle during unstable travel. Thus, the function of the driving stability control system in critical (unstable) situations includes imparting the vehicle behavior that is desired by the driver to the vehicle, within physical limits.
In ESP control systems (ESP=Electronic Stability Program), a pressure requirement for each individual wheel is calculated from the detected instability of the vehicle, and the said pressure requirement is necessary to bring the vehicle back to the course desired by the driver. Yaw torque control ensures stable driving conditions in a cornering maneuver. Different vehicle reference models, e.g. the single-track model, can be relied on for yaw torque control. In ESP control systems, input quantities that result from the course desired by the driver (e.g. the steering angle, the speed, etc.) are always sent to the vehicle model circuit which determines a nominal value for the yaw rate from these input quantities and from parameters characteristic of the driving behavior of the vehicle as well as from quantities predetermined by ambient conditions (coefficient of friction of the roadway, side wind). The said nominal yaw rate is then compared with the actual yaw rate measured. The yaw rate difference is converted into a yaw torque which represents the input quantity of a distribution logic by means of a so-called yaw torque controller or, precisely, a yaw torque control law. The distribution logic itself determines the brake pressure to be applied to the individual wheel brakes in dependence on a brake pressure model. At least the inlet and outlet valves are actuated by a pressure control which converts pressure quantities into valve actuation signals in dependence on the real pressure increase and pressure decrease characteristics in the wheel brakes reproduced in the pressure model. The pressure model receives input quantities required herefor and, based on these and on system parameters, reproduces the pressure that prevails in the brake. More particularly, the pressure model can receive the control signals which influence the brake pressure on the respective brake under review, that is e.g. signals for the inlet valves, the outlet valves, for the hydraulic pump, or similar components. From these signals and from system parameters (for example, line cross-sections, switching characteristics, etc.), the pressure model can reproduce the pressure in the wheel brakes in parallel to the build-up of the wheel pressure so that the control circuit can be closed by outputting the pressure determined in this manner by way of the pressure model.
Prior art systems suffer from the difficulty of taking into consideration the of varying temperatures. The viscosity of brake fluid drops at low temperatures. This changes an input quantity which is taken into account in the pressure model when reproducing the wheel pressure, the pump delivery capacity or the supply volume of the pump, which increases or reduces in dependence on the temperature-responsive viscosity of the brake fluid.
To avoid discrepancies between the wheel pressure reproduced in the pressure model and the actual wheel pressure, it would be desirable to adapt the parameters that are stored in the pressure model or made available to the pressure model, especially the pump delivery capacity.
The design of the present invention, therefore, discloses a method for determining parameters for the viscosity and/or temperature of a brake fluid of a vehicle which is supplied to the wheel brakes by way of a motor-and-pump assembly equipped with actuatable valves and a hydraulic unit or hydraulic block, with which an electronic control unit is associated. The temperature of the hydraulic unit is measured by way of a temperature-sensitive element which connects the motor-and-pump assembly to the electronic control unit, and the parameters are determined by way of the temperature of the hydraulic unit. According to the present invention, the parameters for the viscosity or temperature, respectively, are forwarded to the pressure model as input quantity (quantities) for the reproduction of the brake pressures in the wheel brakes. The parameters for the viscosity or temperature can be determined from the time variation and/or the value of the measured temperature of the hydraulic unit. Advantageously, the temperature-sensitive element, in particular a temperature sensor configured as a thermistor (NTC) or thermally controlled resistor (PTC) based on temperature-responsive resistance variations, is arranged on a preferably electrically pluggable supply element which connects the motor-and-pump assembly to the electronic control unit. The motor-and-pump assembly and the electronic control unit are so attached on opposite sides to the hydraulic unit that the electric supply element interconnects both through a channel in the hydraulic unit. The channel has an inside diameter which is only slightly larger than the outside diameter of the supply element. A safe arrangement or attachment of the supply element in the channel of the hydraulic unit is achieved by way of elastic elements, preferably, spring elements which are arranged on the periphery of the supply element. A temperature sensor is arranged preferably on at least one spring element or several spring elements. A thermal contact between the temperature sensor and the hydraulic unit is established by way of the shaped spring element which projects from the periphery of the supply element. This locates the temperature sensor automatically at a point inside the hydraulic unit during the assembly of the controller (ECU), hydraulic unit, and pump, the said location permitting measurement of the temperature of the brake fluid by way of the hydraulic unit.
In another design, the temperature of the electronic control unit can also be measured by way of a temperature-sensitive element arranged on the conductor plate, and the parameters can be determined by way of the temperature of the electronic control unit.
The temperature sensor which is preferably designed as a thermistor (NTC) is connected to the ground connection of the motor-and-pump assembly. This reduces the number of the sensor connections on the plate of the electronic control unit.
The temperature values determined are forwarded as input quantities to a pressure model which reproduces the actual pressures of the brake pressure in the wheel brakes. The pressure increase and/or pressure decrease characteristic curves of the pressure model are modified in dependence on the temperature values. By means of the parameters supplied to the brake pressure model, parameters stored, calculated or input in the pressure model, such as the pump delivery capacity, the supply volume, as well as values derived from the parameters, are modified or corrected corresponding to the defined parameters for the viscosity or temperature of the brake fluid.
The wheel pressure reproduced on the pressure model is adapted to the actual wheel pressure according to the following relations.
One embodiment of the present invention is illustrated in the accompanying drawings and will be described in detail in the following.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.